CERN Accelerating science

The goal of my master’s thesis is the validation and optimization of the software for the reconstruction of D+s mesons, with the new analysis framework of the ALICE experiment. ALICE studies heavy ion collisions at the CERN LHC to investigate the properties of the Quark-Gluon Plasma (QGP), which can be created in such collisions. In the QGP, quarks and gluons are deconfined over distances much larger than the hadronic size (∼1 fm). Some of the particles produced in heavy ion collisions can be used as “probes” of the QGP because they are produced on short timescales and subsequently they traverse the QGP and can thus help us under- stand its nature. To completely understand the properties of the QGP, it is essential to investigate, as a reference, proton-proton (pp) collisions, in which an extended QGP phase is not created. One should compare the behaviour of the particles under study, both in heavy ion and pp collisions, to understand how their production and kinematics is modified by the QGP formation. Among such QGP probes, the D+s meson, a particle consisting of a charm quark and a strange antiquark, is particularly relevant. D+s mesons live for a short amount of time (cτ ≈ 150 µm) and decay weakly, creating a secondary vertex, which is displaced from the primary vertex, where the proton-proton or nucleus-nucleus collision takes place. My thesis focuses on the D+s decay into a π and a ϕ, with the latter shortly decaying into a pair of K mesons. The secondary vertices are located before the first layer of the ALICE Inner Tracker (ITS 2), the closest detector to the interaction region. Therefore it is only possible to track the D+s decay particles rather than the D+s itself. From the decay tracks, the secondary vertex, where the D+s decayed, can be reconstructed. The produced D+s consist of a prompt component, originating from the hadronization of charm quarks, or from the decay of excited charm hadron states, and a non-prompt component, originating from the decay of a hadron containing a beauty quark. For non-prompt mesons, the displacement of the D+s decay vertex is larger, due to the longer lifetime of beauty hadrons (cτ ≈ 500 µm). The ALICE detector, which has recently undergone a major upgrade, consists of several subdetectors, covering a wide angular range. In my thesis I focus on the ITS 2 and the Time Projection Chamber (TPC), which are used for charged-particle tracking. Particle identification is based on the TPC and the Time Of Flight detectors. The code for the D+s reconstruction is very complex, but it can be summarized into two steps. In the first, triplets of particles that could represent the decay products of a D+s candidate are selected, and the position of the secondary vertex of the D+s candidate is reconstructed. In the second step these candidates go through a selection procedure, identifying those that pass specific quality criteria. In the previous data taking periods, this procedure was carried out in the AliPhysics framework. Now, with the new Run 3 data taking period, a complete restructuring of the analysis code has taken place, leading to the currently used O2 Physics framework. In the first part of my thesis I worked with O2 Physics to optimize the reconstruction of secondary vertices from 3 and 2 decay products, considering various possible candidate particles (e.g. D+ ) and not only D+s . There were three procedures that can be used to compute the common vertex of three and two tracks, and I identified the optimal one that yield the most accurate reconstruction of secondary vertices. In the second part I compared the performance of AliPhysics and O2 Physics, to understand the compatibility of D+s results. After some modifications of the O2 Physics code, I could fully reproduce the results of AliPhysics. The final part of my thesis aimed at finding the best working point for the selection of D+s . In fact, the selection criteria used in prior data taking periods were no longer applicable in Run 3, due to the upgrades to the ALICE detector.